1. Technical Field
The present invention relates to methods and apparatus for manufacturing metal containers in general, and to methods and apparatus for providing a protective coating to an area of the metal containers in particular.
2. Background Information
In the manufacture of metal containers such as small tubs of 0.5 litre to several litres capacity, lugs are provided on either side of the container body as anchor points for the carrying handle. These lugs are attached to the bodies preferably by welding or by some other method of fixing after the bodies have been resistance spot welded; this, however, means that the protective coating or covering applied to the sheet metal prior to fabrication of the containers is damaged and/or that bare metal is left exposed after the attachment process. These areas of damaged coating or bare metal then have to be given a protective coating to prevent corrosion by, or contamination of, the product contained. The invention can of course be applied to any situation where unprotected areas of the kind described above are to be covered with a coating for similar reasons.
In principle, methods which suggest themselves for covering the areas of damaged protective coating or of bare metal are those used to produce large areas of protective coatings on bare metal. In one such method, can seams are coated by a wet lacquering process in which a lacquer is applied to the surface to be coated. Disadvantages of this known method are the long drying times and the solvent vapors given off in the drying phase, necessitating the increasing use, for environmental reasons, of special extraction systems.
In another known method for covering welded can seams which have been exposed by the welding process, electrically charged coating powder is sprayed between lateral shields on to the weld seam, which is carried continuously past the spraying unit and is therefore xe2x80x9cendlessxe2x80x9d, while excess coating powder which accumulates in the shields is continuously extracted. The powder which has been sprayed on is then heated in heating lines, which can be up to 20 meters long, and is thereby cured. This known method, which is sometimes also used in the manufacture of welded tub bodies (to cover the exposed body seam) is, as has been said, continuous, that is to say endless, and is not suitable for treating small areas. Moreover, it is likely that the intact, already coated surface of the containers would be contaminated, which would necessitate subsequent cleaning, or heating of the entire internal surface of the container in order to bind the pulverulent coating material.
Lastly, there is the option of sticking a foil with protective coating material over the damaged areas. This known procedure utilizes an additional substance, namely an adhesive suited to the purpose, and this should be avoided if possible, to avoid further contamination and spoiling of the product contained. For containers intended to be filled with a food product, for example, this would mean that the foodstuff compatibility of the adhesive would have to be proved and/or approval obtained from such authorities as the FDA in the USA, which is a time-consuming and expensive process. In any event, the adhesive would still need to be pronounced completely safe for the individual product to be contained, not least in view of the shelf life of up to several years expected of metal packaging.
The amount of material absorbed in the process of restoring the covering layer is also relevant, as such containers may be manufactured in large numbers, with the result that the consumption of material (in addition to the capital costs) is a critical factor for the production line.
Accordingly the problem which lies at the basis of the present invention is to specify a method and an apparatus which are capable of performing the task with a minimum amount of additional coating material.
This problem is solved by the measures stated in the characterizing part of claim 1. Advantageous configurations of the invention, an apparatus for carrying out the method and a transport system are indicated in further claims.
The invention has the following advantages. Because material is applied only in the region where bare or damaged areas occur, and there is no longer any need for surplus powder to be removed, it becomes possible to cover such areas at small cost in terms of coating material. Furthermore, there is no contamination of the container and no giving off of noxious solvent vapors, as it is possible to work with materials that are already recognized and accepted as appropriate, notwithstanding the fact that the methods used hitherto in the can industry (where lugs may be unknown) basically cannot be applied to the solution of the present problem.